Spring and method



Patented Oct. 31, 1950 SPRING AND METHOD Fredrick Kenneth Bloom, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio Application November 1, 1948, Serial No. 57,761 In France January 10, 1947 7 Claims.

This application is a continuation-in-part of my copending application, Serial No. 575,561, filed January 31, 1945, now abandoned, entitled Spring and Method, and the invention relates to stainless steel springs, and to a method of producing the same.

One of the objects of my invention is the pro- Vision of a simple, reliable and thoroughly practical method of producing stainless steel springs to achieve a high torsion modulus and high resistance to elastic drift coupled with other desired properties including hardness and high elastic limit.

Another object of my invention is the provision of a direct and efficient method of achieving a high torsion modulus and high resistance to elastic drift in hardened stainless steel springs with consequent improvement in elastic limit of the springs.

A further object of my invention is the provision of hardened stainless steel springs, as in the form of leaf or wire springs, which are capable of giving highly satisfactory performance in torsion and in resisting elastic drift under load.

Other objects of my invention in part will be obvious and in part will be pointed out hereinafter.

The invention accordingly consists in the composition of materials, features of products, and in the several steps and the relation of each of the same to one or more of the others Ias described herein, the scope of the application of which Vis indicated in the following claims.

In the accompanying drawing, I present a graph illustrating certain features of my invention, this particularly indicating the effect of cold work on the rate of increase in elastic properties of 16-2 chromium-nickel stainless steel wire.

As conducive to a clearer understanding of certain features of my invention it may be noted that stainless steel is deiined in general terms as a steel which includes carbon ranging up to about 0.5% for relatively 10W-carbon grades and from the latter amount up to approximately 1.20% in high-carbon grades, from about 10% to approximately 35% chromium, either no nickel or nickel in amounts ranging up to 10% or more, occasionally supplemental additions of manganese, silicon, cobalt, copper, molybdenum,

tungsten, vanadium, columbium, titanium, sulphur, and the like, for special purposes, and a balance which is substantially all iron.

There are many possible uses for stainless steel (Cl. 14S-12.4)

springs which make it important because of operations to be performed or functions to be achieved that the springs exhibit resistance to elastic drift; elastic drift being a small, slow, continued stretch of a spring, or contraction as the case may be, as a result of prolonged or repeated stress over a, period of time, although the stress is below the elastic limit as commonly termed. Then, either with or apart from the need for resistance to elastic drift, there are many possible uses for stainless steel springs in which rigidity displayed by the metal to torsionally applied loads enters as an important consideration; that is, a high torsion modulus, or in other words a high modulus of rigidity is required for best performance.

In way of illustration, there are precision instruments employing springs which should for best results possess high torsion values and high resistance to drift to ensure exact instrument readings. Along in the same category are contact breaker springs for marine or industrial uses which also are kept under pressure for long periods of time desirably without relaxation of tension on compression by creeping or drifting during the accumulation of time. Then, in way of further illustration, there are springs for refrigerator valve controls which are exposed to heat exchange coupled with torsional or elastic drift e'ects. At times too, as in the examples noted, and as in the case of springs in valves of chemical conduits on tanks or illustratively in outdoor uses, the resilient products or springs are exposed to more or less severe corrosive attack, thus making corrosion resistance an active problem. y

On the other hand many springs in the prior art, including work hardened 18-8 chromium nickel stainless steel springs, possessing desired corrosion-resisting properties are lacking of satisfactory torsion values or rigidity under torsired spring properties including high elastic limit, hardness and high tensile strength.

Referring now more particularly to the practice of my invention, I nd that stainless steel springs characterized by outstanding improvements in torsion modulus and resistance to elastic drift are achieved by a special combination of heating and Working operations from hardenable steels which comprise carbon ranging from 0.05% or less up to approximately 1.20%, chromium between and 20%, either no nickel or nickel ranging from fractional percentages up to about 6%, small amounts of one or more supplemental elements such as copper er molybdenum for special purposes if desired, and the remainder substantially all iron. The process, or special operations, which I employ for producing stainless steel springs from steel of the character described includes providing the steel in hardened condition, cold-working the same to an extent ranging from something more than slight. notably 50%, up to an extensive amount, notably 85%, While in the hardened condition, and following the cold-working with a tempering treatment. The improved torsion values and drift resistance are found in the resulting stainless steel spring products where the cold work essential to the process are achieved by cold drawing. Along with the improved torsion values and resistance to drift or creep, there also is a consequent and highly valuable improvement in elastic limit notable in the finished springs.

I prefer to provide my stainless steel springs of quench-hardenable chromium-nickel steels which have a chromium content of about 11.5% to 18% along with a nickel content of 1.25% t0 2.5% and even up to 3.5%, and contain between about 0.05% and 0.5% carbon. These springs, particularly those of higher chromium content of about to 18% chromium and more particularly those which include about 16% chromium and approximately 2% nickel, offer maximum resistance to corrosion together with high rigiditl7 when torsionally stressed, and thoroughly effective and reliable resistance to elastic drift under load, all after treatment of the steel in accordance with my process involving heat treatments and cold work.

Of the straight-chromium varieties of stainless steel springs which I provide are those of relatively cheap quench-hardenable steel having a chromium content of about 11.5% to 13.5% and a carbon content of about 0.05% to 0.15%. A higher carbon variety of straight-chromium stainless steel springs which also are worthy of speciiic note among those produced in accordance with the process described herein have a carbon content of about 0.20% to 1.20% and a chromium content of approximately 16% to 18%.

As illustrative of the practice of my invention, steel of the composition noted, in billet form, is hot-worked as by hot-rolling, to a heavy wire size. Following this, the hot-worked wire is annealed to soften, then pickled to clean and de-scale the surface, coated with a suitable lubricant and subsequently cold-drawn. Where necessary, as where considerable reduction in size is required, the annealing, pickling, coating and cold-drawing steps are repeated. The cold-drawn wire is reduced to the approximate size desired and this, in accordance with the practice of my invention, is hardened by quenching from a temperature ranging between 1750 F. and 2050 F. Substantially a full hardness is had. In a further instance I start with iiat strip or wire of the bardenable stainless steel in an annealed, pickled and cold-rolled or drawn condition, which is subjected to a hardening heat above critical temperature and this is quenched to achieve the hardness. The quench-hardening treatment, such as in the practices just referred to, is the irst step in my process of obtaining my stainless steel springs having a high torsion modulus and high resistance to elastic drift as well as a high elastic limit.

Usually, I follow the quench-hardening step with a pickling operation to clean up the metal surface, especially to remove heat scale. The pickling treatment preferably is achieved by immersing the hardened stainless steel in a molten caustic hydride bath as for about 20 minutes; this advantageously being followed by dipping the metal in a cold nitric acid bath to whiten and thus protect the metallic surface. The caustic hydride bath is rapid in action and causes a minimum reduction in size of the metal during the pickling treatment. An alternative and further illustrative pickling treatment which I employ involves subjecting the hardened stainless steel to treatment in a caustic permanganate bath as for approximately 6 hours; this, for example, being followed by dipping the metal in a cold nitric bath to whiten. The pickling of course may be omitted entirely, as when the foregoing hardening step amounts to a bright-hardening treatment or for some other reason the metal presents a clean surface.

As a further step essential to the provision of my stainless steel springs having high torsion modulus and high resistance to drift, I subject the hardened stainless steel to cold-working such as to rolling or drawing. When cold-rolling is employed in providing the necessary cold-work. conveniently it may serve the additional important function of reducing the metal to a gauge which approaches or substantially corresponds to that of a desired nished spring product, such as a at spring. A standard rolling mill is satisfactory for accomplishing the rolling operation. Likewise, when relying in part at least upon cold-drawing for the cold work required, the metal illustratively is drawn in rod, or wire form to substantially finished gauge of a desired wire or coil spring. The reduction in cross-sectional area of the metal as a result of the coldworking may range from 40% up to 85% or more, say 95%, or even 98%, for extremely ne wire sizes such as those approaching .004 diameter. With finished wire sizes up to about .020 diameter, the cold-reduction does not exceed The severe cold-drawing of the metal, for example, leads to a high measure of tensile strength of the nal spring products.

Usually during the completion of the coldworking treatment in my process, the stainless steel is worked or formed, as by coiling or bending, into a spring product having a substantially finished contour. I subject the cold-worked steel to a tempering treatment, which consists in heating the metal to a temperature between 300 F. and 1000 F., preferably above 400 F. but below 900 F. or even 800 F., for approximately one hour, although in applications where no surface discoloration whatever is permissible, I iind a temperature under 450 F. or even under 400 F. is preferable.

In the early stages of the tempering treatment. a large gain is had in torsion modulus, elastic drift resistance and elastic limit of the metal. such as in the iirst ten minutes of tempering. A short gesamter tempering period, therefore, may be used to advantage-if desired.- After the' tempering treatand a torsion modulus of elasticity of about 11.55v to 1150x106 p. s. i. The modulus compares with that of high-carbon musicwire springs and is better than work-hardened v518-8 chromiumnickel steel springs. Y

The critical character of the cold work employed in my process and product is illustrated inthe accompanying drawing wherein thechange in the percent increase in the elastic limit is shown for variousamounts of cold reduction, the increase inelastic limit rising rapidly with coldreduction beginning with a reduction of some 40% and continuing to a reductionof 851% or more.y

To illustrate a specific example of the practice of my process, an annealed, pickled and colddrawn roughed stainless steel Wire (0.097 inch diameter) for the production of coil springs is given a hardening heat in an open red continuous furnace at 2000 F. while maintaining a wire speed of 45 feet per second. On leaving the furnace the wire is quenched in air for hardening and as hardened illustratively has an ultimate strength of about 235,000 p. s. i. and an elastic limit of approximately 90,000 p. s. i. The Wire then is pickled for about 20 minutes in a molten caustic hydride bath and thereafter is whitened in cold nitric acid. The wire diameter at this point illustratively is 0.095 inch. I then dip the pickled wire in a molten lead bath to provide a surface lubricant for later cold-drawing and spring winding or coiling operations. In the procedure of cold-drawing, the cooled wire is passed successively through say four dies, each reducing the cross-sectional area sufficiently to give for example an 0.047 inch nal diameter, (the cold reduction thus being 75.4%), an ultimate strength of 324,100 p. s. i., an elastic limit of 100,000 p. s. i. and a 180 at bend without fracture. The lead coated, cold-drawn wire then is formed into springs, as by a spring manufacturer, illustratively on a coiling machine. After the coiling operation the lead coat is removed such as by dipping the springs in a nitric acid solution. My tempering treatment then is applied to the cold-worked stainless steel springs which in this instance illustratively consists in heating the metal at 800 F. for 30 minutes. By the particular sequence of hardening, cold-drawing, coiling and tempering treatments just referred to the spring products provided illustratively have an ultimate tensile strength of 320,000 p. s. i., an elastic limit of 282,-

' 800 p. s. i. and a modulus of elasticity in torsion of 11.'72 106 p. s. i. and a maximum yield point stress in torsion of 200,600 p. s. i. These properties will be understood to compare favorably with those of steel music Wire and to be considerably superior to those of conventional 18-8 chromiumnickel stainless steel springs.

The stainless steel springs which I achieve through the practice of my process, whether in the form of wire or flat springs as of compression, extension, torsion, elliptical or cantilever type, are particularly suitable for uses Where high torsion values or resistance to elastic drift, either or both, in combination with high elastic limitand one or more of strength,`ductility and corrosionresistance are important. Among the spring products which I provide are springs for precision.= instruments such as bombsights', speed indicator-s1` gyroscopes and Vnavigation instruments in which' very low elastic drift properties, hitherto available onlyin copper-beryllium and a 'few other alloys, are required for exact and proper functioning of these instruments; gun springs, lock springs, governor springs and the like which are exposed to: Y corrosive'gases; and springs for watches, clocks,- business vrmachinesor dispensing machines in whichv spring control or operation is required as undertorsion or inthe' presence of corrosive inuences or loads which demand resistance to elas-r tic drift. In afurth'er. category of products which Iiprovide varewire,rod or spring shapes such as windshield wiper shafts, fishing rods and knives' as with long blades, inwhich bending, pulling or. twisting takes place and in which vit is important' that'the metal return to original shape without substantial lossv of properties or dimensions.

. Thus it will be seen that there is provided in' my invention a method of providing stainless;

steel springs to achieve desired high torsion Values and resistance to elastic drift in combination with high elastic limit, wherein the objects hereinbefore noted, together with many thoroughly practical advantages are successfully achieved. It will also be seen that the process of my invention is successfully practiced with readily available materials and equipment.

As many possibe embodiments may be made of my invention, and as many changes may be made in the embodiments hereinbefore set forth, it will be understood that all matter described herein is to be interpreted as being illustrative and not as being a limitation.

I claim:

1. In producing stainless steel flat or wire springs comprising 10% to 20% chromium and from fractional percentages up to 1.20% carbon, with carbon increasing with chromium and having high torsion modulus and high resistance to elastic drift, the art which includes, quenchhardening from a temperature between 1750 F. and 2050 F., cold-'working of the hardened metal to an extent between a reduction of 50% and and tempering the same at a temperature of 300 F. to 900 F. i

y2. In manufacturing stainless steel resilient springs displaying increased torsion modulus and improved resistance to elastic drift, the art Which includes, providing steel wire containing 11.5% to 18% chromium, 1.25% to 3.5% nickel and 0.05% to 1.25 carbon, quench-hardening from a temperature between 1750 F. and 2050 F., coldworking the hardened steel to a reduction in area of 40% to 85%, and tempering the same.

3. In manufacturing stainless steel resilient products displaying increased torsion modulus and improved resistance to elastic drift, the art which includes, providing steel in quench-hardened condition containing 15% to 18% chromium, 1.25% to 3.5% nickel and 0.05% to 0.5% carbon, cold-working of the hardened steel to an extent between 50% and 85%, and tempering the hardened and cold-worked steel at temperature of 400 F. to 800 F.

4. In manufacturing stainless steel resilient springs up to about .020" finished diameter and displaying increased torsion modulus and improved resistance to elastic drift, the art which includes, providing steel wire containing 15% to 18% chromium, 1.25% to 3.5% nickel and 0.05% to 0.5% carbon, quench-hardening the same from a temperature of 1750 F. to 2050 F.,

7 cold-Working the hardened lsteel wire Ato a 'reduction in area of 40% to 85 the `Wire so worked initially being of such size as to `Yield the diameter noted, Yand tempering the same Yat a temperature of 400 F. to 800 F.

5. In manufacturing stainless steel resilient springs of about .020" -to about .004 nshed diameter and displaying increased torsion modulus and improved ,resistance to elastic drift, the art which includes, providing vsteel wire containing 15% to 18% chromium, 1.25% to 3.5%v nickel and 0.05% to 0.5% carbon, quench-hardening the same from a temperature Lof 1750 F. to 2050 F., cold-Working the hardened steel Awire to a. reduction in area of 85% to 98%., kthe Wireso worked initially being of such size as to yield the ldiameter noted, and tempering the same ata temperature of 400 F. to 800or F.

6. Predominantly martensitic stainless steel springs compris-ing from fractional percentages up to 1.25% carbon and 10% to 20% chromium and with carbon increasing With chromium,

quench-hardened, subjected to 4cold-Working to an extent between areduction of 40% and 85%, and tempered to achieve a high torsion modulus in combination with high resistance to elastic drift.

7. Predominantly martensitic stainless steel springs comprising from 0.05% to 0.5% carbon. 15% to 18% chromium and 1.25% to 3.5% nickel,

. quench-hardened, cold-reduced in area between 0 Number REFERENCES CITED The following references yare of record in the file of this patent:

UNITED STATES PATENTS Name Date 2,266,952 Bloom Dec. 23, 1941 

1. IN PRODUCING STAINLESS STEEL FLAT OR WIRE SPRINGS COMPRISING 10% TO 20% CHROMIUM AND FROM FRACTIONAL PERCENTAGES UP TO 1.20% CARBON, WITH CARBON INCREASING WITH CHROMIUM AND HAVING HIGH TORSION MODULUS AND HIGH RESISTANCE TO ELASTIC DRIFT, THE ART WHICH INCLUDES, QUENCHHARDENING FROM A TEMPERATURE BETWEEN 1750*F. AND 2050*F., COLD-WORKING OF THE HARDENED METAL TO AN EXTENT BETWEEN A REDUCTION OF 50% AND 85%, AND TEMPERING THE SAME AT A TEMPERATURE OF 300*F. TO 900*F. 